(a) Field of the Invention:
This invention relates to a novel and improved thermoplastic resin sheet. More specifically, this invention relates to thermoplastic resin sheets substantially free of thermal expansion and shrinkage and/or having a low degree of optical anisotropy, and a production process therefor. This invention is also concerned with applications of these thermoplastic resin sheets.
(b) Description of the Prior Art:
The term "thermal shrinkage" as used herein means the dimensional change which a thermoplastic resin sheet undergoes when heated. The degree of dimensional change can be measured as a thermal shrinkage in accordance with, for example, Japanese Industrial Standard K 6734.
It has been known that sheets for use in stickers, label bases, liquid crystal displays (LCDs) or the like are required to have high levels of dimensional accuracy and cannot be used effectively if they develop dimensional changes, curling or the like upon application.
Thermoplastic resin sheets have hitherto been produced by the calendering technique or extrusion technique. However, the levels of the thermal shrinkage of sheets produced in accordance with these techniques have usually been about .+-.3% or, when their production was effected under such conditions as to minimize the levels of their thermal shrinkage, have still been about .+-.1% or so. If such conventional thermoplastic resin sheets are employed in applications requiring high levels of dimensional accuracy, such as labels, stickers, liquid crystal displays and the like, as mentioned above, they develop such problems when used, for example, in labels or stickers, as colors may be mismatched or patterns may be distorted upon printing, their glue may project out to smear their corresponding bases, or they may become readily separable or peelable in the course of use. Therefore, there has been a long standing demand for the development of thermoplastic resin sheets having much smaller levels of thermal shrinkage.
Optical recording has in recent years become wide-spread because it permits high density recording, fast recording, non-contact readout of records and excellent durability. Light-transmitting resin sheets are used in optical recording media in order to protect their record-bearing layers. These resin sheets are required to have low degrees of optical anisotropy for the following reasons: Polarized light such as laser beams or the like is used for optical recording. If a resin sheet has optical anisotropy, the quantity of light to be measured, for example, upon readout of a record varies, thereby producing considerable noise and considerably reducing accuracy.
Therefore, it is desired to provide sheets having low levels of optical anisotropy as light-transmitting resin sheets suitable for use in optical recording. As a production method for sheets having a low level of optical anisotropy, in other words, there has already been known the so-called solution casting method it has already been known to dissolve a thermoplastic resin in a solvent and to cause the resultant solution cast on a substrate. Even if this method is used, each of resulting sheets has only a very small area that is substantially free from optical anisotropy. Therefore, they cannot be used as large-sized optical recording materials. Further, their production costs are high. Under the circumstances, they can thus be used only in some extremely limited application fields. On the other hand, sheets available in accordance with the calendering or extrusion techniques have high levels of optical anisotropy and thus cannot be used as optical recording materials at present.